High-speed telecommunications switches accept optical inputs and produce optical outputs but internally involve almost entirely electronic switching. Single-mode optical fibers are the technology of choice for long-haul transmission of information because they have very wide bandwidth, low attenuation, and low dispersion, making it possible to transmit information at very high bit rates (1010 b/s) over long distance (105 m) without the need of repeaters. Digital electronics, however, is the technology of choice for switching. Digital integrated circuits can switch connections in less than a nanosecond, and 105 to 106 digital gates can be incorporated on a single integrated circuit facilitating construction of the logic that controls a fast switch on a cycle by cycle basis.
FIG. 1 shows a block diagram of prior art telecommunications switch. Inputs arrive on single-mode optical fibers 101. In a typical application, these fibers carry a serial bit stream at 2.5 Gbits/s (OC48) and are formatted using SONET (synchronous optical network) framing. The signal on each input fiber 101 is converted to electrical form by optical-to-electrical (O/E) converters 102. These converters demultiplex the data stream, converting the 2.5 Gbits/sec serial stream into a 16-bit wide stream at 156 Mbits/s. This electrical version of the input stream 103 is then input to an electronic switch 1-4. The switching fabric of the internet routers described in published PCT patent application number PCT/US98/16762 is an example of such a switch. The switch extracts individual packets or cells (depending on the protocol) from the SONET frames (or other framing) of the incoming streams on each input and forwards each packet or cell to the output to which it is addressed. At the output, the packets or cells are encapsulated in a SONET frame and output electronically. This electrical output stream 105 is then output to an electrical-to-optical converter (E/O) and the optical output stream 107 drives the long-haul fiber to the next telecommunications switch.
It is attractive to use an electronic switch to direct optical signals because it is very easy to build logic and memory electronically, and very difficult to realize these functions optically. The switch requires logic to examine the contents of packets and determined where they are to be routed, and the switch requires memory to buffer packets according to a quality-of-service.